It has been known for many years to provide four wheel vehicle driving such as disclosed by U.S. Pats. Nos. 1,095,916 to Nichols; 1,129,643 to Christensen; 1,735,984 to Straussler; 1,932,786 to Johnson; 2,065,767 to Thompson; 2,136,692 to Johnson; and 2,915,133 to Kraus. Such four wheel driving provides increased traction and thus the capability of better vehicle control in situations where two wheel driving does not provide adequate traction.
Conventional vehicle drivetrains for providing four wheel driving include a transfer case which has a housing separate from the vehicle transmission that is driven by the vehicle engine. This type of transfer case has an input driven by the output of the transmission and has a pair of outputs respectively connected to the front and rear axles. In part time four wheel driving, one output of the transfer case is continually connected to the associated axle to provide continuous driving while the other output is selectively connected and disconnected so as to thereby provide either two or four wheel driving of the vehicle. In full time four wheel driving, both outputs of the transfer case are continuously connected to the associated axles. With part time transfer cases, the front and rear wheel outputs are often locked with respect to each other such that use thereof during continual on the road driving is not possible due to torque buildup in the drivetrain between the front and rear axles as a result of the different paths of travel of the front and rear wheels. However, such locked four wheel driving is advantageously used in off the road conditions or other lower speed driving requiring high traction. With full time four wheel driving, an interaxle differential of the transfer case between the front and rear axles is often utilized to permit the front and rear axles to operate at different speeds as required by their different paths of travel. To prevent one axle from spinning when one of its wheels loses traction, it is also conventional to incorporate a viscous clutch with the differential of the transfer case such that the axles can rotate with respect to each other only at limited speeds so as to thereby insure continuous four wheel driving.
Front wheel driving of vehicles has increased to a dominant extent during the last ten years or so. Such front wheel driving is conventionally provided by a front engine that drives a combination transmission and axle normally referred to as a "transaxle". The engine and transaxle are conventionally located adjacent the front of the vehicle and advantageously provide increased front wheel loading that improves the driving traction of the front wheels driven by the transaxle. Multiple gear paths of the transaxle drive a differential thereof which is in the same oil bath as the gear paths, and axially aligned outputs of the differential respectively drive the left and right front wheels at a speed determined by the gear paths selected by the transmission through a conventional manual or automatic selection. Vehicle power plants including an engine and transaxle are relatively economical in addition to providing the traction and other advantages associated with front wheel driving.